1. Field of the Invention
The present invention relates to adjustable oscillator.
2. Brief Description of Related Developments
In electronic devices adjustable oscillators are used especially when two or more different frequencies are required and the implementation of these frequencies with separate oscillators is not reasonable or possible. Furthermore, adjustable oscillators are used for producing frequencies in a particular frequency range. If the stability of the frequency to be produced is not very significant, it is possible to form such an adjustable oscillator as a voltage-controlled oscillator which has no back coupling. However, in many cases it is necessary to attain certain frequency stability. Thus, the frequency produced by an adjustable oscillator at a given time has to be located in a relatively accurate manner on a desired frequency, and the frequency must not vary significantly. Thus, a feedback loop, such as a phase locked loop (PLL), and an accurate reference frequency are used in connection with the voltage-controlled oscillator. By means of a frequency divider located in the feedback of the phase locked loop it is possible to attain desired frequencies which can be kept in the desired value by means of a phase comparator. Such oscillators are necessary especially in communication applications in which a certain frequency range is allocated for a wireless communication device, in which range the transmitter and the receiver have to operate. This is necessary for example to prevent the wireless communication device from interfering with other wireless communication devices in the vicinity. In addition, possible frequency variations may cause excess modulation, which causes interferences in the communication.
In a voltage-controlled oscillator (VCO) the frequency is adjusted by changing the control voltage of the oscillator. The change in the control voltage generates a change in the resonance frequency in the resonance circuit of the oscillator, which changes the frequency produced by the oscillator. This change in the resonance frequency can be advantageously accomplished in such a manner that the capacitance and/or inductance of the resonance circuit is/are adjusted. Typically, the adjustment means used in voltage-controlled oscillators is a voltage-controlled capacitor (capacitance diode, varactor) and/or a group of capacitors composed of several capacitors, wherein such capacitors are selected from this group of capacitors, by means of which the desired oscillator frequency can be produced.
In a phase locked loop the output frequency is produced by means of a voltage-controlled oscillator. The signal produced by this voltage-controlled oscillator is conducted to a frequency divider, which divides the oscillation signal with a divider set in the frequency divider. Thus, the output of the frequency divider contains a signal whose frequency is the frequency produced by the voltage-controlled oscillator divided with the divider. This signal is conducted to a phase detector, and the reference frequency is conducted to the second input branch of the same. The phase detector produces a voltage proportional to the phase difference of the signal of the frequency divider and the reference signal, which voltage is filtered in a filter and thereafter conducted to the input of the voltage-controlled oscillator. Thus, this voltage produced by the phase detector can be used for adjusting the frequency of the voltage-controlled oscillator until the frequency of the voltage-controlled oscillator has been set in its target value. In this situation, the reference signal and the signal produced by the frequency divider have substantially the same phase. This reference signal is produced for example by means of a crystal-controlled oscillator, wherein the phase locked signal can be utilized to produce a stable output frequency, whose frequency can be adjusted.
In electronic devices voltage-controlled oscillators are typically composed of discrete components. However, it is a constant aim to reduce the size of electronic devices, such as wireless communication devices. Thus, the aim is also to integrate the voltage-controlled oscillator in an integrated circuit. Thus, problems are caused for instance by the fact that the capacitances of the capacitors used for example in the resonance circuit of the oscillator vary in different integrated circuits, and changes in temperature result in significant variations in the capacitance. Thus, the frequency control range of the oscillator can shift from the desired one, which complicates the process of producing the correct frequencies in the phase locked loop. The center frequency of the oscillator can for example shift aside to such a degree that the desired frequency range cannot be successfully produced any more, but some of the lower or higher frequencies remain outside the control range. A prior art solution is to adjust the capacitances of the integrated circuit at the manufacturing stage by means of a laser beam, but this method is relatively expensive, and it can be conducted only once, wherein the tuning cannot be performed during the use of the electronic device. Temperature compensation cannot be taken into account either in this laser adjustment. Another alternative for amending this problem is to increase the gain of the voltage-controlled oscillator, i.e. a larger output frequency range is attained in the same control voltage range. Thus, however, the effect of the disturbances on the signal produced by the oscillator is also increased. Yet another alternative is to provide the voltage-controlled oscillator with a wide control range of the control voltage. Especially in portable devices in which the aim is to reduce the operating voltage, such solutions would cause the problem that in order to produce the control voltage of the voltage-controlled oscillator, a voltage converter should be used, by means of which higher voltages can be produced from the lower operating voltage. This, however, increases the power consumption of the electronic device and requires additional components, which is not desirable as the devices become smaller in size and the cost requirements become tighter.
It is an aim of the present invention to provide an adjustable oscillator and a tuning method for an adjustable oscillator, in which the tuning can also be conducted during the use of the electronic device. The invention is based on the idea that to adjust the oscillator, a tuning stage is conducted, in which the aim is to set the center frequency of the adjustable oscillator to a desired point, substantially in the middle of the control range of the control voltage.
Thus, in one aspect the present invention is directed to a method for adjusting an oscillator. In one embodiment the method comprises tuning an adjustable oscillator, in which oscillator at least one resonance circuit is used, and the frequency of the oscillator is adjusted by changing the resonance frequency of the at least one resonance circuit by means of a control signal, for which a minimum value and a maximum value are selected. At least one target value is selected for the control signal. The frequency of the adjustable oscillator is set to substantially correspond to the target value. The value of the control signal and the target value are compared, and when the value of the control signal is substantially different from the target value, a tuning signal is produced to change the resonance frequency of the at least one resonance circuit.
In another aspect the invention further relates to an adjustable oscillator. In one embodiment the adjustable oscillator comprises at least one resonance circuit, means for controlling the frequency of the oscillator by changing the resonance frequency of the at least one resonance circuit by means of a control signal, for which a minimum value and a maximum value are selected, and means for tuning the oscillator which comprise means for producing at least one target value of the control signal, means for setting the frequency of the adjustable oscillator to substantially correspond to the target value and comparing the value of the control signal and the target value. The oscillator also comprises means for producing a tuning signal to change the resonance frequency of the at least one resonance circuit when the value of the control signal is substantially different from the target value.
In a further aspect the invention also relates to an electronic device. In one embodiment the electronic device comprises an adjustable oscillator which contains at least one resonance circuit, means for controlling the frequency of the oscillator by changing the resonance frequency of the at least one resonance circuit by means of a control signal, for which a minimum value and a maximum value are selected, and means for tuning the oscillator, which comprise means for producing at least one target value of the control signal, means for setting the frequency of the adjustable oscillator to substantially correspond to the target value and comparing the value of the control signal and the target value. The electronic device also comprises means for producing a tuning signal to change the resonance frequency of the at least one resonance circuit when the value of the control signal is substantially different from the target value.
The present invention shows remarkable advantages when compared to solutions of prior art. When the method according to the invention is applied, the control range of the adjustable oscillator can be set to a desired point, wherein for example in communication applications all desired frequencies are attained by means of the oscillator. Tuning according to the invention can also be conducted during the use of the electronic device, and in the tuning it is possible to take into account the possible effects of the temperature on the frequency of the oscillator, wherein the electronic device functions in a more reliable manner. Another advantage attained by means of the invention is that the adjustable oscillator can be implemented in an integrated circuit, wherein the size of the electronic device can be reduced and the manufacturing costs can be decreased. Furthermore, as separate laser tuning is not necessary, the electronic device according to the invention can be manufactured at smaller expenses and within a shorter period of time when compared to a situation where solutions of prior art are used. In that case external measurement devices are not required either.